1. Field of the Invention
The present invention relates to a switched reluctance motor (SRM) drive device to drive an SRM. More particularly, the present invention relates to a passive converter for an SRM drive device capable of controlling a 3-phase SRM of the passive converter.
2. Description of the Related Art
In general, a switched reluctance motor (SRM) requires a switching controller, and both of a stator and a rotor of the SRM have a salient pole structure. Particularly, since a winding is wound around only the stator, but the rotor has no winding or a permanent magnet, the SRM has a simple structure. Due to the structural characteristic of the SRM, the SRM has remarkable advantages when the SRM is manufactured. In addition, while the SRM generates great torque with superior driving performance like a DC motor, it is unnecessary to frequently perform the maintenance of the SRM. Since the SRM has superior characteristics in an amount of torque per unit volume, efficiency, the rate of a converter, and so forth, the application fields of the SRM are gradually increased.
FIG. 1 is a block diagram schematically showing a single-phase SRM drive device according to the related art.
Referring to FIG. 1, the single-phase SRM drive device includes a rectifier and smoothing circuit 102, a motor driver 103, and a position sensor 105. The rectifier and smoothing circuit 102 rectifies and smoothes AC voltage, which is applied from a supply voltage 101 (or AC supply voltage), into DC voltage. The motor driver 103 receives the DC voltage from the rectifier and smoothing circuit 102 and a control signal from a micro-processor 106 to drive a motor 104. The position sensor 105 detects the position and the speed of the motor 104 to output a detection signal to the micro-processor 105.
The rectifier and smoothing circuit 102 rectifies and smoothes AC voltage input from the supply voltage 101. The rectified and smoothed voltage is supplied to the motor driver 103, and the motor driver 103 supplies the voltage to the motor 104 according to the control signal from the micro-processor 106. The micro-processor 106 receives the detection signal generated from the position sensor 105 of detecting the rotational speed and the phase of the motor 104 to control the motor driver 103, and controls the voltage supplied from the motor driver 103 to the motor 104.
FIG. 2 is a circuit diagram showing one example of the motor driver 103 of the single-phase SRM drive device.
The motor driver 103 of the single-phase SRM drive device includes a DC link capacitor 201, upper and lower switching elements 202 and 203, a motor winding 206, a first diode 204, and a second diode 205. The DC link capacitor 201 supplies DC voltage into which input AC voltage is rectified and smoothed. The upper and lower switching elements 202 and 203 are connected with the DC link capacitor 201 in parallel. The upper and lower switching elements 202 and 203 are connected with each other in series and are turned on/off according to a driving signal output from a switch driver to output a gate driving signal used to forwardly or reversely rotate the motor 104 according to a position signal of a rotor of an SRM 207. The motor winding 206 generates torque according to the on/off operation of the upper and lower switching elements 202 and 203. The first diode 204 is connected between one terminal of the upper switching element 202 and one terminal of the lower switching element 203. The second diode 205 is connected between opposite ends of the upper and lower switching elements 202 and 203.
Accordingly, the upper and lower switching elements 202 and 203 are turned on during a predetermined period of time according to the positions of the stator and the rotor of the SRM 207, so that a current path of the DC link capacitor 201, the upper switching element 202, the motor winding 206, and the lower switching element 203 is formed, thereby applying the DC voltage, which is obtained through smoothing, to the motor winding 206. Therefore, magnetic force is generated from the stator of the SRM 207 to attract the rotor, so that the SRM 207 is rotated. If both of the upper and lower switching elements 202 and 203 are turned off when the SRM 20 rotates, phase current, which has been applied to the motor winding 206, flows to the supply voltage 101 through the first diode 204, the motor winding 206, the second diode 205, and the DC link capacitor 201.
As described above, the SRM drives the motor 104 by supplying or cutting off voltage to the motor 104 according to an on/off state of the upper and lower switching elements 202 and 203 constituting the motor driver 103. Regarding control signals applied to the upper and lower switching elements 202 and 203, as shown in FIG. 1, if the position sensor 105 detects the phase of the motor 104 to provide a detection signal to the micro-processor 106, the micro-processor 106 performs PWM (pulse width modulation) using the detection signal from the position sensor 105, such that the control signals control the on/off operation of the upper or lower switching element 202 or 203 according to a duty ratio of the PWM.
FIG. 3 is a circuit diagram showing another example of the motor driver 103 of the single-phase SRM drive device.
The motor driver 103 shown in FIG. 3 includes a pair of DC link capacitors 301 and 302, a motor winding 305, a first diode 306, and a second diode 307. The DC link capacitors 301 and 302 supply DC voltage obtained by smoothing input AC voltage. The upper and lower switching elements 303 and 304 are connected with the paired DC link capacitors 301 and 303 in parallel. The upper and lower switching elements 303 and 304 are connected with each other in series and are turned on/off according to a driving signal output from a switch driver to output a gate driving signal used to forwardly or reversely rotate the motor 104 according to a position signal of a rotor of the SRM. The motor winding 305 generates torque according to an on/off operation of the upper and lower switching elements 303 and 304. The first diode 306 is connected between one end of the upper switching element 303 and one end of the lower switching element 304. The second diode 307 is connected between opposite ends of the upper and second switching elements 303 and 304.
Hereinafter, the operating procedure of the motor driver 104 of the single-phase SRM drive device having the above structure will be described.
When supply voltage (AC voltage) of 220V is applied to the motor driver 104 from an external voltage source, the DC voltage of the paired DC link capacitors 301 and 302 approximates about 310V. If the upper and lower switching elements 303 and 304 are turned on, excitation voltage is applied the motor winding 305, so that phase current of the motor winding 305 is gradually increased. Thereafter, if the upper and lower switching elements 303 and 304 are turned off, the phase current of the motor winding 305 is decreased due to demagnetization voltage having a magnitude identical to DC 310V of the excitation voltage.
In the conventional SRM drive device, since excitation voltage to apply current to the motor winding 305 is identical to demagnetization voltage to remove the excitation voltage, it takes too much time (t) to remove the current from the motor winding 305. If the phase current stays in the motor winding 305 for a long time, a stator attracts a rotor, called counter-torque, so that problems may occur in the SRM operating at a high speed.
In the motor driver 103 of the SRM drive device, since excitation voltage and demagnetization voltage are limited to DC link voltage, phase current required at high-speed operation is difficult to build up, so that negative torque is generated and output power is reduced due to tail current.
In order to solve the problems, various boost converts have been suggested.
For example, the DC link voltage may be increased by employing a boost converter and a buck-boost converter, thereby improving the utilization of torque and increasing the output power.
However, the boost converter and the buck-boost converter additionally require an inductor, a diode, a capacitor, and a power switch. Accordingly, the price of the converters is increased, and a complicated boost voltage control method is required.
In addition, series and parallel passive boost converters have been suggested. Although the two passive boost converters have a simple structure, demagnetization current is charged to an additional boost capacitor, so that effective boost voltage higher than DC link voltage is applied. However, since the boost voltage depends on recovered energy, a controllable speed range is restricted.